LOS ANGELES , California -LRB- CNN -RRB- -- Smoking in youth-rated movies has not declined despite a pledge two years ago by Hollywood studios to encourage producers to show less `` gratuitous smoking , '' according to an anti-smoking group .

The American Medical Association Alliance has been trying to get movie studios to make smoking-free films .

The American Medical Association Alliance , pointing to research that big-screen smoking leads teens to pick up the tobacco habit , called for an R rating for any movie with smoking scenes .

The head of the group that gives U.S. movies their ratings , however , said the smoke has been clearing from youth-rated movies , a result of the film industry 's sensitivity to the issue .

The alliance , the medical association 's advocacy arm , launched a summer campaign this week aimed at publicly shaming studios into making smoke-free films .

`` Research has shown that one-third to one-half of all young smokers in the United States can be attributed to smoking these youth see in movies , '' said Dr. Jonathan Fielding , head of the Los Angeles County Public Health Department .

Fielding cited another study that he said `` found that adolescents whose favorite movie stars smoked on screen are significantly more likely to be smokers themselves and to have a more accepting attitude toward smoking . ''

The Motion Picture Association of America , the industry group that issues ratings and parental guidance for U.S. films , added smoking scenes as a factor in ratings two years ago , but Fielding said it has not made a difference .

`` In all , 56 percent of the top box office movies with smoking released between May 2007 and May 2009 were youth-rated films -- G , PG or PG-13 , '' he said .

Joan Graves , who chairs the Motion Picture Association 's movie rating committee , offered her own statistics , based on all of the 900 films rated each year , not just the top movies included in Fielding 's numbers .

The association has given no G ratings in the past two years to a movie with smoking , Graves said .

Overall , 55 percent of the movies rated in the past two years showed some smoking , but 75 percent of those with smoking scenes were given R ratings , Graves said . Twenty-one percent were rated PG-13 and the remaining 5 percent were PG , she said .

A G movie is deemed suitable for all audiences , while a PG rating is a signal to parents that a film may include some material they might consider inappropriate for children . PG-13 indicates a stronger warning that some material may be inappropriate for children under 13 .

No one under 17 can be admitted to see an R movie without a parent or guardian .

American Medical Association Alliance President Sandi Frost used as her chief example of a movie with `` gratuitous smoking '' this month 's blockbuster `` X-Men Origins : Wolverine , '' which was rated PG-13 `` for intense sequences of action and violence , and some partial nudity . ''

`` Millions of children have been exposed to the main star of the film , Hugh Jackman , with a cigar in his mouth in various scenes , '' Frost said . `` I 'm willing to bet that not one child would have enjoyed that movie or Mr. Jackman 's performance any less if he had n't been smoking . ''

A spokesman for Twentieth Century Fox , the studio responsible for the Wolverine movie series , said Jackman 's cigar was never lit and it was limited to just two scenes .

In one scene , the cigar is shot out of his mouth , prompting Jackman 's Wolverine character to suggest its loss would lead to clean living -- an anti-smoking statement -- the studio spokesman said .

He said that while the Wolverine character has a cigar in his mouth in almost every panel of the comic book series , producers made `` a conscious decision '' to limit the cigar in the movie .

The American Medical Association Alliance , hoping to draw studio executives ' attention , hired a mobile billboard to drive around the major studios this week .

`` The billboard shows a teenage girl asking the question , ` Which movie studios will cause me to smoke this summer ? ' '' Frost said .

The alliance will keep an online scorecard throughout the summer to count `` how many tobacco impressions each studio delivers to G , PG and PG-13 audiences , '' she said .

`` At the end of the summer , whichever studio has delivered the most tobacco impressions to youth audiences will be named in a billboard that will run outside of their headquarters , '' she said .

Motion Picture Association of America spokeswoman Angela Martinez said the group `` is very sensitive to the concerns of parents about the purpose of the rating systems . ''

`` It 's reflective of society , '' Martinez said . `` It 's really a tool for parents to help determine what their kids see . ''

They began factoring smoking scenes into the ratings two years ago as `` a reflection of changes in society and health concerns , '' she said .

`` Smoking is rated like all the other factors , including violence and sex , '' she said .

Fielding said it should be absolute -- and not just a factor .

`` Any movie with smoking should be rated R , '' he said . `` And if they worry about an R rating hurting their profits , then they should work with studios to remove smoking from films that hurt youth . ''

Graves , whose committee makes the decisions , indicated such a zero-tolerance policy would not be accepted .

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American Medical Association Alliance wants films with smoking to be rated R

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AMAA says that kids who watch smoking often turn to habit

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Hollywood rating organization says smoking in movies has declined